This invention relates to the synthesis of aliphatic aromatic phosphine oxides by a five step process beginning with a phosphinic acid. In particular, it relates to the synthesis of octylphenyl phosphine oxide by that process.
Secondary phosphine oxides have a general formula: ##STR1##
Secondary phosphine oxides having one aliphatic group and one aromatic R group ("mixed" phosphine oxides) are of particular interest because they have been found to be very useful in making aliphatic aromatic N,N-dialkylcarbamoylmethyl phosphine oxides, which are useful in extracting lanthanides and actinides.
Phosphine oxides can be synthesized from phosphinic acids, which have the general formula: ##STR2## The synthesis of a phosphine oxide from the corresponding phosphinic acid, where both R groups are aliphatic or both R groups are aromatic, does not present any particular difficulties. But processes for synthesizing mixed phosphine oxides from mixed phosphinic acids are difficult and expensive.
One process is to react the mixed phosphinic acid with triethylamine and trichlorosilane in toluene to produce the mixed phosphine followed by oxidation to produce the mixed phosphine oxide (see E. P. Horwitz et al., Sol. Ext. and Ion Exch., 4, 449-494, 1986.): ##STR3## However, during the addition of trichlorosilane a voluminous, thick white precipitate is obtained and it is not possible to stir the reaction mixture unless large amounts of solvent are used. Difficulties are also encountered with stirring during the hydrolysis. As a result, yields are erratic and poor. Further work showed that the triethylamine can be eliminated, which reduced the stirring problems. However, if the toluene solution of the acid was below 20.degree. C. during the addition of trichlorosilane, the reaction mixture would freeze into a sticky solid which made stirring extremely difficult. Yet the reaction could not be heated above 30.degree. C. during trichlorosilane addition because unreacted trichlorosilane boils at about 31.degree. to 32.degree. C.
It is also possible to produce the mixed phosphine oxide from the mixed phosphinic acid by means of a Grignard reaction. However, the use of a Grignard reagent requires handling ether, which is highly flammable. Special equipment and facilities are required which are not economical for making small amounts of materials or for making materials that do not justify the expense.